Acid-etching of sound enamel results in an increase in the surface area and a more reactive surface. Since caries-like lesions possess an intact surface overlying the body of the lesion, it has been shown in pilot studies that typical etching patterns may be produced on the surfaces of caries-like lesions in vitro. The purpose of this study is to investigate the role of the acid-etch technique in caries formation, remineralization and progression. The techniques which will be used are qualitative and quantitative polarized light microscopy, scanning electron microscopy and chemical analyses for calcium, fluoride and phosphate. The study is divided into six, individual experiments. The first two will investigate the effects of acid-etching and sealing of sound enamel and caries-like lesions on lesion formation and progression. The next two experiments are directed toward evaluating the effect of the acid-etch technique used in conjunction with topical fluoride agents and calcifying fluids on lesion remineralization and progression. The morphology of crystals in sound enamel, etched enamel and caries-like lesions which have undergone various treatment regimens will be characterized in the fifth experiment. The final experiment will determine the concentrations of fluoride, calcium and phosphate present in surface zones from control, etched, remineralized and progressed caries-like lesions. These results will be compared with those obtained from sound enamel. The long range objective of this study is to develop a clinically applicable method to prevent caries formation and to remineralize existing lesions. The study will add to the basic knowledge regarding the acid-etch technique and the mechanisms of the carious process.